Ionizing radiation dose can be estimated by analyzing a magnitude of an ionization current resulting from an ionization chamber sensing ionizing radiation. In some applications that have a strict requirement of measurement uncertainty (e.g., having an error tolerance that is generally less than 3%), such as a dose area product (DAP) meter, a desired dynamic range of the ionization current may range between, for example, 1 pA and 100 nA.
When a transimpedance amplifier and an analog-to-digital converter are simply used to convert the measured weak current into digital codes, the measurement precision is influenced by a plurality of factors, such as high resistance resistors of the transimpedance amplifier, alternating current (AC) voltage coupling, thermal noise, the bit number limit of analog-to-digital conversion, and periodic and/or non-periodic noises that may be coupled to electronic circuits and that may result in large periodic and/or non-periodic oscillation of an analog voltage signal converted from the weak current by the transimpedance amplifier. Although the periodic noise may be filtered out using techniques such as spectrum analysis, filters, etc., oscillation of the analog voltage signal may exceed upper and/or lower limits of the digital codes, causing the final output to be unreliable.